There are a number of different painting solutions available. I am assuming that it is a mild steel structure . Look at Devoe Marine Coatings. Basically it is sandblast, then epoxy prime with a urethane topcoat, should last for a number of years unless you are submerged...The navy uses some really serious epoxy primers that I believe are made by Pro-line.

By your consideration of HDG, the structure is NOT immersed.  Therefore there are many protective coating systems that will provide acceptable performance.  Devoe has no special products I'm aware of that would outperform those from PPG, Carboline, Sherwin-Williams, International Paint, ...etc. etc.

The key is to have the steel fabricator apply the paint system in the shop.  The fabricators usually have a favorite paint supplier to work with.  As long as this supplier is a reputable company (who also stands behind the application, so the fabricator and paint maker don't point fingers if it fails), the likely system would be an organic zinc primer (4 - 5 mils DFT) with a high-build urethane topcoat (4 - 5 mils DFT) (epoxies are good too but tend to chalk over time).

Some touch up after erection will improve the chances of a 15 year life, which is  reasonable expectation for this system.
 

Try and find your nearest 'Zenga' Stockist, This stuff is made in belgium, and is used extensively by the Belgian Navy, I specify its use here in Petrochemical Plant in Scotland. Must be the nearest thing to 'Cold Galvanising'
It is actually a very high zinc content in the dry film paint, it actually looks like Galvanizing.

Hugh, what ever the paint system, surface prep is the key.